Quick Answer:
If you open the cabinet and smell something rotten, start by checking for wet spots, drips, or food trapped under the sink. Often the cause is a forgotten spill, a wet sponge, or waste caught in the trap. If those aren’t the issue, sniff around the trap and where the drain goes through the wall to tell whether the smell is from plumbing or from the cabinet itself.
Why This Happens
Rotten smells under a sink usually come from organic material breaking down or from sewer-type gases coming up through the drain. Common causes:
- Leftover food, a dropped piece of fruit, or a wet sponge sitting on the cabinet floor.
- Food, grease, or hair trapped in the P-trap that sits under the sink.
- Loose or dried-out slip-joint washers or seals letting odor through even with no obvious water leak.
- A gap at the point where the drain pipe passes through the wall, allowing sewer gas to escape into the cabinet.
Step-by-Step What to Do
Step 1 — Look for visible moisture and leaks
Open the cabinet and inspect under the trap and at every slip joint. Look for moisture, drips, or staining on the pipes and cabinet floor. Pay close attention to the area directly under the trap and where pipes connect.
Step 2 — Check the cabinet floor for forgotten food or wet items
Feel the cabinet floor for sticky or damp spots. Remove any sponges, dishcloths, food wrappers, or containers. Even a small spill that sat for a few days can create a strong rotten smell.
Step 3 — Sniff to locate whether it’s plumbing or cabinet odor
If the cabinet is dry and clean, bend down and sniff near the trap and where the drain pipe goes into the wall. If the odor is strongest at the pipe penetration or around slip joints, it’s likely plumbing-related. If the smell is strongest on the cabinet floor or back wall away from pipes, it’s likely from spilled food or something stored there.
Step 4 — Clean the trap and nearby pipe if needed
Place a small bucket under the trap, loosen the slip nuts, and remove the trap to empty trapped water and debris. Rinse the trap with hot, soapy water and a brush. Reassemble the trap, making sure the washers seat correctly. If you’re not comfortable doing this, skip to the “When to Call a Professional” section.
Step 5 — Tighten slip joints carefully and test
Hand-tighten the slip nuts; do not overtighten plastic fittings. After reassembly, run water for a minute and check again for leaks or odor. If the smell persists after cleaning and tightening, see Odor returns after cleaning for troubleshooting ideas.
Step 6 — Consider what was recently disposed of
If you recently ground food, especially meat or fibrous scraps, the sink can retain a strong odor. For context-specific tips, see Smell after grinding meat.
What Not to Do
- Don’t assume ‘no drip means no problem’—loose slip-joint washers can leak odor without visible water; if you smell sewer gas strongly or feel unsure reseating joints, a plumber is appropriate.
- Don’t use harsh chemicals without knowing what’s already in the trap—mixing cleaners can produce dangerous fumes.
- Don’t ignore the smell for days; the source may be a health or pest issue (rotten food or a dead animal).
- Don’t try to force or overly tighten plastic fittings; that can crack fittings and create a real leak.
When to Call a Professional
- If you smell strong sewer gas (sour or rotten-egg type) and can’t find the source.
- If you’re uncomfortable removing or reseating the trap or slip-joint washers.
- If tightening or cleaning doesn’t stop the odor, or if you find recurring leaks or wet spots behind the cabinet wall.
- If you suspect a dead animal inside a wall cavity—call pest control or a plumber with wall-access experience.
Safety Notes
- Ventilate the area while you work; open windows or run a fan to remove odors and fumes.
- Wear gloves when handling trap contents—there can be bacteria and rancid material.
- Do not mix household cleaners. If you use a bleach solution to clean a non-plumbing surface, rinse well and avoid using ammonia or other cleaners that could react.
- If you detect a strong gas smell and feel dizzy or ill, leave the home and call for help.
Common Homeowner Questions
- Could the smell be a dead animal? — Yes; small animals can get into cabinets or wall cavities. Use gloves and remove the source, or call pest control if you can’t access it safely.
- Will pouring bleach down the drain fix it? — Bleach may temporarily mask odors but won’t remove trapped food or fix faulty seals; cleaning the trap and seals is more effective.
- How long before the smell goes away after cleaning? — If you remove the source and clean the trap, the smell often fades within a few hours to a day with good ventilation.
For more related articles, see the Food Waste Odors & Bio-Buildup hub.